The rear disc brakes started in the 2001 model year, but those so-named trucks began manufacture in mid 2000. If you got an early 2001 that was built I think before June or so, you got rear drums. They used up the remaining axles with drums, it was a running change.
Also as part of that change they eliminated the hood liner, hood/cowl seal with the flap, rear wheelwell liners, rear-seat cupholders, halo ring, black antenna, and the visor mirror for the driver. Some trucks have some of these, some have none depending on when it was built and when they ran out of that specific part.
My 2001. 5 has rear discs as well as everything else I listed except the driver mirror, and was built in August of 2000.